Membrane-based separation has proved to be an efficient technology for oxygen enrichment from air, especially for applications where only moderate purity is needed. Typical application of oxygen-enriched air include combustion, ozone generation, aerobic wastewater treatment, medical respiration, diving and high altitude flight breathing. For a given membrane, the separation performance is affected by process design and configuration. Conventional membrane gas separation is operated in a steady-state fashion where both the feed pressure and the permeate pressure are maintained at constant levels. As the transmembrane pressure difference is the driving force for permeation, the most efficient way to enhance permeation is to increase the pressure difference across the membrane. This can be done in two ways: feed pressurization and permeate evacuation. Current technologies, however, are difficult to employ in practice.